Catadioptric optical system having multilayer filter reflector

ABSTRACT

A catadioptric optical system for improving the quality of the image and the transmittance of the optical system. The invention is an improvement on the Cassegrain and Newtonian optical systems or any other optical systems wherein the secondary mirror partially obscures the primary mirror. The secondary mirror in these optical systems is provided with a multilayer, angle sensitive, filter coating, with the coating being designed to pass to the primary mirror substantially all radiation received along the optical axis and to reflect substantially all radiation received at angles of incidence other than along said optical axis. With this arrangement radiation normally blocked by the secondary mirror is passed to the primary mirror which results in an effective increase in the transmittance of the optical system and an improved modulation transfer function for the optical system.

"1' WW4 E United States X g Fischer et al. Y #0 8 '7 Y CATADIOPTRICOPTICAL SYSTEM HAVING MULTILAYER FILTER REFLECTOR [75] Inventors: RobertE. Fischer, C arlisle; Wayne Knapp, Sudbury; George R. Wirtenson,Act0n.all of Mass.

[73] Assignee: Itek Corporation, Lexington, Mass.

FOREIGN PATENTS OR APPLICATIONS 111 3,748,020 July 24, 1973 PrimaryExaminer-John K. Corbin Attorney Homer 0. Blair, Robert L. Nathans etal.

[57] ABSTRACT A catadioptric optical system for improving the quality ofthe image and the transmittance of the optical system. The invention isan improvement on the Cassegrain and Newtonian optical systems or anyother optical systems wherein the secondary mirror partially obscuresthe primary mirror. The secondary mirror in these optical systems isprovided with a multilayer, angle sensitive, filter coating, with thecoating being designed to pass to the primary mirror substantially allradiation received along the optical axis and to reflect substantiallyall radiation received at angles of incidence other than along saidoptical axis. With this arrangement radiation normally blocked by thesecondary mirror is passed to the primary mirror which results in aneffective increase in the transmittance of the optical system and animproved modulation transfer function for the optical system.

4 Claims, 6 Drawing Figures 1,257,192 2/1961 France 350/201 I l0 ANGLE B78 MULT/LAYEI? mm? I6 ANGLE B lMAGE l4 BACKGROUND OF THE INVENTION Thepresent invention relates generally to the field of obscured opticalsystems, and more particularly pertains to an improvement in suchoptical systems whereby radiation normally blocked by the secondarymirror in each of these systems is passed to the primary mirror.

Most reflecting optical systems of the prior art require an obstructionto the primary mirror in the form of a secondary mirror to reflectconverging radiation from the primary mirror to a convenient imagelocation. This optical arrangement has two primary disadvantages. First,the amount of radiation reaching the image plane is reduced from that ofan unobscured system of the same F/number. To regain the loss of theobscured radiation optical designers are required to use lower F/numbersystems. For example, in an F/4 optical system wherein the secondarymirror blocks 50 percent of the incoming radiation, the illumination atthe image plane is equivalent to an F/5.66 unobscured system. ln orderto achieve the same illumination at the image plane as an unobscured F/4system, an F/2.83 system would be required. The primary mirror of anF/2.83 system is 1.4 times the diameter of the unobscured F/4 system.The larger F/2.83 system requires the primary mirror to be larger andheavier, and therefore more expensive. Further, geometric aberrationsare more difficult to correct in a lower F/number system, and thisresults in a degradation of the final optical performance of the system.Second, the obscuration by the secondary mirror reduces the modulationtransfer of the optical system and thus reduces image quality.

Some prior art optical systems employ a beam splitting folding mirror of50 percent transmission over its full aperture. Such a system containsno central obstruction. However, typically 75 percent or more of theincident radiation is lost in the beam splitter.

It would be desirable to have a telescope wherein radiation normallyblocked by the secondary mirror of the telescope is permitted to pass tothe primary mirror.

SUMMARY OF THE INVENTION In accordance with a preferred embodiment, acatadioptric optical system is disclosed for improving the quality ofthe image produced by the optical system. In the catadioptric opticalsystem the secondary mirror, which is positioned at a given anglerelative to the optical axis of the optical system, has an anglesensitive filter coating applied to it. The angle sensitive filtercoating is designed to pass substantially all radiation incident on thesecondary mirror along the optical axis of the telescope at the givenangle. The angle sensitive filter coating reflects substantially allradiation incident on it at angles other than said given angle. Thisarrangement results in incoming radiation along the optical axis of theoptical system being passed through the secondary mirror to the primarymirror, and then being reflected by the primary mirror back to thesecondary mirror for further reflection to an image plane. Thecatadioptric optical system results in an increase in the effectiveF/number of the optical system for a given diameter of focal length ofthe primary mirror and also an improvement in the modulation transferfunction of the optical system. Further, the preferred embodimentprovides an optical system having a reduced weight and size mirrorsystem without degraded optical performance as would be expected in aprior art system of equivalent weight and size which does not utilizethe teachings of this invention.

Although the two illustrated embodiments of this invention aretelescopic optical systems, the teachings of this invention extend toall optical systems wherein a secondary mirror normally partiallyobscures a primary mirror.

BRIEF DESCRIPTION OF THE DRAWINGS two telescopes, one constructed in theprior art fashion and the second embodying the teachings of thisinvention.

FIG. 6 shows a Cassegrain optical system constructed in accordance withthe teachings of this invention.

DESCRIPTION OF A PREFERRED EMBODIMENT FIGS. 1 and 2 illustraterespectively Newtonian and Cassegrain optical systems constructed inaccordance with the teachings of the prior art. As illustrated by FIGS.1 and 2, reflecting optical systems of the Newtonian or Cassegrain typeshave an obstruction in the form of a secondary mirror 10 to reflectconverging radiation from a primary mirror I2 to a convenient imageplane 14. The right sides of FIGS. 1 and 2 illustrate thev circle ofradiation obstructed by the secondary mirrors. The radiation blocked bythe secondary mirrors result in an effective decrease in thetransmittance of each optical system and a degraded modulation transferfunction for each optical system.

FIG. 3 illustrates one embodiment of this invention wherein theteachings of this invention are applied to a Newtonian optical system.In the optical system illustrated in FIG. 3, the secondary mirror 10extends across the entire aperture of the primary mirror, and has anangle sensitive multilayer filter coating I6 thereon. A narrow bandpassmultilayer filter is capable of reflecting or transmitting differentamounts of nearly monochromatic radiation as a function of the angle ofincidence of the radiation. In this illustration, all angles will bereferenced to the optical axis 15 of the optical system. Radiationincident on the secondary mirror at an angle a and along the opticalaxis of the optical system passes through the secondary mirror 10 to theprimary mirror 12. The angle sensitive coating 16 is designed totransmit substantially all radiation incident thereon at an angle a andreflect substantially all radiation incident thereon at angles otherthan a. Thus, much of the obstruction normally presented by the see- Iondary mirror is now removed from the optical system.

+ B. A small amount of radiation incident on the optical axis I5 will bereflected by the primary mirror 12 back along the optical axis to thesecondary mirror and will pass through the secondary mirror 10 as it isincident upon the secondary mirror 10 at angle a. However, substantiallyall radiation incident on the secondary mirror at angles greater than orless than alpha will be reflected to image plane 14. In the embodimentillustrated in FIG. 3 the secondary mirror 10 was shown across theentire system aperture for purposes of illustration. A more practicalembodiment would omit the annular area and only include the central area18 in the secondary mirror as only the central area 18 reflectsradiation to the image plane 14.

One embodiment of this invention was designed for a configuration asillustrated in FIG. 3 but with annular area 20 of the secondary mirroromitted. This embodiment was designed with an Fl 1 .5 primary mirror andan angle a of 45. Utilizing techniques which are well known in the art,and which are explained in Military Standardization Handbook OpticalDesign, DOD-MIL-I'IDBK 141 (FSC-6650), Oct. 5, i962, and particularlysection 20 thereof, a 23 layer filter was designed having thetheoretical transmission and reflection properties shown in FIG. 4. InFIG. 4 it may be seen that approximately 95 percent of radiationincident on the filter at an angle of exactly 45 will be transmitted bythe filter. At angles of incidence of one half degree greater or lessthan 45, 85 percent of the incident radiation will be reflected, and atangles of incidence 2 w greater or less than 45 greater than 99.9percent of the incident radiation will bereflected. It should be notedthat this graph is for radiation at one particular wavelength. Ingeneral, multilayer filters having characteristics similar to thatillustrated in FIG. 4 may be designed for any desired wavelength ofinterest and any desired optimum angle.

The following table compares the transmission and reflection propertiesof a typical prior art optical system with the designed embodimentutilizing the teachings of this invention.

neglecting reflection Iones which would be present in both systems.

FIG. 5 compares the modulation transfer function (MTF) curves of atypical prior art optical system with the designed embodiment employingthe teachings of this invention. The MTF, shown as the ordinate in FIG.5, is basically a measure of image quality with 1.0 representing aperfect image. The normalized frequency, shown as the abscissa in FIG.5, is a measure of the spatial frequency of lines in the image.Referring to FIG. 5, it may be seen that at most frequencies anobstructed system shows a significant drop in MTF, as might be expected.

The designed embodiment is a small field, nearly monochromatic opticalsystem. Such a system would be particularly useful in active, low lightlevel systems utilizing a monochromatic laser source. However, it shouldbe understood that the teachings of this invention have applicabilitybeyond small field, nearly monochromatic systems depending upon thedesign of the angle sensitive filter coating.

FIG. 6 illustrates one embodiment of this invention wherein a Cassegrainoptical system utilizes the teachings of this invention. In thisembodiment, the angle sensitive filter coating is designed to transmitsubstantially all radiation incident at an angle of and to reflectsubstantially all radiation incident at angles of other than 90. Withthis embodiment a small radiation loss will be incurred by radiationwhich is incident on the optical axis as it will pass through thesecondary mirror 10 to the image aperture and will not be reflected bythe primary mirror 12. One advantage of the system illustrated in FIG. 6is that since the radiation is passed through the filter at 90,polarization effects, which might otherwise have to be taken intoaccount, may be disregarded. While several embodiments have beendescribed, the teachings of this invention will suggest many otherembodiments to those skilled in the art.

We claim:

1. A Catadioptric telescope for improving the quality of the imageproduced by the telescope and comprismg:

a. a primary mirror placed along the optical axis of the optical systemfor gathering radiation;

b. a secondary mirror for reflecting to an image plane radiationreflected by said primary mirror onto the secondary mirror;

c. said secondary mirror being positioned at a given angle relative tothe optical axis of the optical system and thereby being positioned atsaid given angle relative to incoming radiation along the optical axis;

d. said secondary mirror having an angle sensitive multilayer filtercoating thereon, said angle sensitive filter coating passingsubstantially all radiation incident thereon at said given angle andreflecting substantially all radiation incident thereon at angles otherthan said given angle whereby radiation normally blocked by saidsecondary mirror will be passed through said secondary mirror to saidprimary mirror and be reflected by said primary mirror back to saidsecondary mirror for further reflection to said image plane, therebyincreasing the effective transmittance of the optical system for a givendiameter and focal length of the primary mirror and improving themodulation transfer function of the optical system.

2. Apparatus as set forth in claim 1 wherein said optical system is aCassegrain optical system wherein said given angle is approximately 90.

3. Apparatus as set forth in claim 1 wherein said optical system is aNewtonian optical system.

4. Apparatus as set forth in claim 3 wherein said given angle isapproximately 45.

1. A Catadioptric telescope for improving the quality of the imageproduced by the telescope and comprising: a. a primary mirror placedalong the optical axis of the optical system for gathering radiation; b.a secondary mirror for reflecting to an image plane radiation reflectedby said primary mirror onto the secondary mirror; c. said secondarymirror being positioned at a given angle relative to the optical axis ofthe optical system and thereby being positioned at said given anglerelative to incoming radiation along the optical axis; d. said secondarymirror having an angle sensitive multilayer filter coating thereon, saidangle sensitive filter coating passing substantially all radiationincident thereon at said given angle and reflecting substantially allradiation incident thereon at angles other than said given angle wherebyradiation normally blocked by said secondary mirror will be passedthrough said secondary mirror to said primary mirror and be reflected bysaid primary mirror back to said secondary mirror for further reflectionto said image plane, thereby increasing the effective transmittance ofthe optical system for a given diameter and focal length of the primarymirror and improving the modulation transfer function of the opticalsystem.
 2. Apparatus as set forth in claim 1 wherein said optical systemis a Cassegrain optical system wherein said given angle is approximately90*.
 3. Apparatus as set forth in claim 1 wherein said optical system isa Newtonian optical system.
 4. Apparatus as set forth in claim 3 whereinsaid given angle is approximately 45*.